Television reception is very often handicapped by multi-path signal propagation conditions which cause groups of the same signal to arrive at the reception point at different times, leading to the presentation of ghost images in the reproduced picture. Double or multiple images are often the result of these undesirable propagation conditions. These ghost images or echoes are typically faint but often objectionable replica of the original or main path signal. Other propagation defects are also often present. These include response (frequency) degradation, short term (less than one microsecond) reflections, group delay errors, random noise and non-random interferences.
The prior approaches to the problem of reducing ghost images have not been very successful, since those prior approaches proceeded on a theory of completely eliminating the unwanted ghost images. Complete elimination of ghost images remains a most difficult technical achievement. Systems and methods for partial ghost image reduction are known in the prior art. These systems have usually relied upon a single sample of the ghost image in order to set the time delay and gain factor for recursive filters.
Examples of prior approaches include: Thedick, "Adaptive Multipath Equalization for T.V. Broadcasting", IEEE Trans. on Consumer Electronics, May 1977, pp. 175-181; Yamamoto, Yamaguchi, and Miyata, "Ghost Reduction System for Television Receivers", IEEE Trans. on Consumer Electronics, Vol CE-23, No. 3, August 1977, pp. 327-333; Makino, Sakurai, Iwasawa, Ohnishi, Murakami and Miura, "A Fully Automatic Ghost Canceller", IEEE Trans. on Consumer Electronics, Vol. CE-24, No. 3, August 1978, pp. 267-271; Murakami, Ohzeki and Ogi, "A Digitalized Automatic Ghost Canceller", IEEE Trans. on Consumer Electronics, Vol. CE-25, August 1979, pp. 555-561; Ciciora, Sgrignoli, and Thomas, "A Tutorial on Ghost Cancelling in Television Systems", IEEE Transactions on Consumer Electronics, Vol. CE-25, February 1979, pp. 9-44; Goyal, Armfield, Geller and Blank, "Performance Evaluations of Selected Automatic Deghosting Systems for Television", IEEE Transactions on Consumer Electronics, Vol. CE-26, February 1980, pp. 100-120; Makino, Murakami, Sakurai, Ohnishi and Obara, "A Novel Automatic Ghost Canceller", IEEE Trans. on Consumer Electronics, Vol CE-26, August 1980, pp. 629-637; Murakami, Iga and Takehara, "Ghost Clean System", IEEE Trans. on Consumer Electronics, Vol. CE-29, August 1983, pp. 129-133; Yip and Kongable, "Advanced Digital Television System", IEEE Trans. on Consumer Electronics, Vol. CE-32, No. 4, November 1986, pp. 743-753; and, Murata, Kazumi, and Kondo, "Practical TV Ghost Canceller Using Two-Stage CCD Transvessal Filter", 1983 IEEE Intern. Conference on Consumer Electronics, Digest of Technical Papers, pp 136-137.
Therefore, a hitherto unsolved need has existed for a practical ghost image reduction control system which is technically feasible at relatively low cost.